It is well known in the treatment of effluent, and more particularly of sewage, to carry out an oxidization reaction using ozone, the ozone being dissolved in the effluent to be oxidized or in some of the sludge from the biological reactor or coming from the clarifier, using an ozonization reactor often called an ozonization contactor. Sewage treatment plants are also known which comprise at least one biological reactor, at least one clarifier and at least one ozonization reactor, in this order or in another. The present invention applies more particularly to this type of plant. It should be recalled that if the ozonization reactor treats water coming from the clarifier, it is sometimes necessary to include an additional treatment step of filtration (over sand) between the said clarifier and the said ozonization reactor, this being done so as to remove the suspended matter from the effluent and thus improve the effectiveness of the ozonization treatment.
After being brought into contact in the ozonization reactor with the effluent to be treated or with some of the sludge from the biological reactor or coming from the clarifier, the effluent gas which escapes from the reactor contains a certain amount of residual ozone. It is necessary to avoid discharging an effluent gas having too high an ozone content into the atmosphere.
To avoid such a discharge, it has been envisaged to recover the residual ozone for the purpose of recycling it at the head of the contactor. Thus, WO-95/05340 describes a water purification process using ozone, in which the undissolved residual ozone is collected in order to be reused in the water treatment. Likewise, EP-A-0,047,925 describes a process and an apparatus for the treatment of water using ozone, in which the effluent gas, which contains undissolved oxygen and residual nitrogen, is recovered and recycled into the ozonization reactor. Furthermore, Patent FR-A-2,490,208 describes a process in which the ozonized gas coming from an ozonization reactor is injected directly into the biological treatment tank, without a prior step of stripping the residual ozone and without the implementing apparatuses being described. Finally, Patent Application FR 97/09882 describes a process and an apparatus for the purification of sewage, comprising an additional treatment of the sludge by ozonization. The gaseous effluent from the ozonization reactors is collected so as to reuse it for treating the sewage or any other liquid resulting from the treatment of this sewage. Claims 12 to 14 of this prior application rely on prior destruction of the ozone before its reuse or reutilization. However, such recycling requires a considerable consumption of energy, given that the effluent gas containing the residual ozone has to be pressurized before it is recycled.
Under these conditions, an operation of systematically destroying the residual ozone is more generally carried out. In this regard, reference may be made to the publication "Memento Technique de l'Eau [Water Technical Memorandum]", 50.sup.th anniversary issue 1989, second edition, published by Degremont, pages 907-909. Of course, this destruction has the drawback of removing, without being used, a not insignificant amount of ozone, which increases the costs.